Films of polymeric liquid crystalline material are well known. GB 2 324 382 (U.S. Ser. Nos. 09/059,247 and 09/484,259) discloses a liquid crystal film with homeotropic alignment. With the described method, nematic and smectic A liquid crystals can be quickly aligned on plastic substrates, whereby roll to roll coating is applicable. The use of an aligning layer comprising a surfactant being fixed in a matrix of a polymeric liquid crystalline material is described. According to an alternative embodiment, the substrate is coated with aluminium or sufficiently smooth Al2O3 to achieve a homeotropic alignment of the liquid crystal material. The resulting uniformly homeotropic aligned films can be used in electrooptical displays, optical compensation layers or as active switching layer.
The use of birefringent films as security devices has been reported in prior art. A hot stamping foil for security applications comprising a liquid crystal material is known from GB 2 357 061. The liquid crystal layer is applied onto a reflective layer, which exhibits on its opposite surface a hot melt adhesive. The liquid crystal layer is protected by a lacquer. The whole arrangement of layers is carried by a support layer. A wax layer between the support layer and the lacquer layer enables a release of the birefringent layer arrangement by applying heat. The liquid crystal material is a nematic or smectic material with a planar, tilted, splayed or homeotropic orientation. The liquid crystal layer comprises a linear or crosslinked liquid crystal polymer with macroscopically uniform orientation. Metal films or flakes are used as a reflective layer. The hot stamping foil can be applied on substrates to authenticate and prevent counterfeiting of documents of value, e.g., of banknotes, credit cards or ID cards. The transmission or reflection effect of the polarized light by the birefringent material can be seen using linear polarizers. Incorporation of dichroic dyes leads to additional color effects.
The use of uniformly aligned nematic liquid crystals in security applications has a major drawback. Due to the uniform alignment only one colour is seen when viewed through a polarizer. This effect could be copied by using any reflective background and a layer of a birefringent material.
A solution of this problem was proposed by the GB 2 330 360 (U.S. Pat. No. 6,217,948), which describes a polymer film with a helically twisted molecular structure exhibiting at least two maxima of a reflection wavelength. The film is obtained by polymerizing a thermochromic mesogenic composition at varied temperatures. Since the temperature is changed during polymerization, the thermochromic polymerizable composition undergoes a change of the helical pitch and therewith of the reflection maximum. The various pitch lengths are fixed by the polymerization reaction and a polymer film is obtained that exhibits, depending on the type, degree and speed of temperature variation, a range of reflection colours. Thus, a disadvantage of this method is the requirement of a precise control of the temperature and its change during the polymerization process
An alternative approach is disclosed in the GB 2 328 180 relating to a security device comprising a birefringent film having regions of different thickness, such that when the material is viewed through a polariser, a pattern is exhibited. The birefringent film is, e.g., a stretched polymer film like a PET film. The pattern is achieved, e.g., by forming recesses in the birefringent film, or by bonding the film to a higher melting point layer as supporting layer and reducing the thickness or destroying the birefringence in selected regions of the birefringent film. Destruction of the birefringence is achieved, e.g., by melting, hot stamping, thermal printing or laser writing. Reduction of the film thickness is achieved, e.g., by removing parts of the film or by laser writing. The higher melting point supporting layer then holds the partially destructed birefringent film in place. GB 2 328 180 further reports that a reflective layer can be applied to the birefringent film after provision of the pattern, e.g., by metallisation or printing of a reflective material.
The device according to GB 2 328 180 has several drawbacks. For example, its manufacture is complicated and requires a series of manufacturing and processing steps like forming the pattern on the birefringent film, lamination to a high melting point supporting layer and application of the reflective layer. The methods of forming the pattern are also complicated and require additional technical effort and equipment, like the use of high temperatures or lasers. Furthermore, stretched plastic films used as birefringent layers as described in GB 2 328 180 generally exhibit only moderate birefringence. Therefore, usually a high film thickness is required to achieve a satisfying optical effect. The film thickness is further increased by the use of the supporting layer. Thus, GB 2 328 180 reports devices with a combined film thickness of the birefringent and supporting layer of about 15 μm. Such a high thickness is critical especially for applications in the security sector, like, e.g., security markings to be applied on documents of value or banknotes.